Photoresists are photosensitive films used for transferring an image formed therein to an underlying layer or substrate. A layer of a photoresist is formed over a substrate and generally a layer of a material to which the image is to be transferred. The photoresist layer is then exposed through a photomask to a source of activating radiation where the photomask has some areas that are opaque to such radiation and other areas that are transparent. A photoinduced chemical transformation results in the areas exposed to the activating radiation which allows for the development of a relief image therein.
Photoresists can be either positive-tone or negative-tone. Generally, negative-tone photoresists undergo a crosslinking reaction within those portions of the photoresist layer that are exposed to activating radiation. As a result, the exposed portions become less soluble than unexposed portions in a solution used to develop the relief image. In contrast, for positive-tone photoresists, the exposed portions of the photoresist layer become more soluble, in a developer solution, than the portions unexposed to such radiation.
As microelectronic devices, such as integrated circuits, employ smaller and smaller device structures to effect their function, the need for photoresist compositions capable of resolving such device structures becomes increasingly important. While the ability of a particular photoresist composition to resolve a particular device structure is a function of many factors, one such factor is control of the difference in the dissolution rates of exposed and unexposed portions of the photoresist. While this factor has been studied previously, the thrust of such study has generally been to develop additives that might serve to increase the difference in the dissolution rate of such exposed and unexposed portions in a developer solution. However, such additives can affect other properties of the photoresist composition in a less than desirable manner, for example by increasing the optical density of the composition at the working wavelength of the activating radiation, and perhaps of more import, merely increasing the difference in the dissolution rates of exposed and unexposed regions does not always control the differential dissolution rate.
Thus it would be desirable to provide photoresist compositions with controlled differential dissolutions rates. That is to say, the difference between the dissolution rates of exposed and unexposed portions of the photoresist is controllable. It would also be desirable for such photoresist compositions to encompass a polymeric material that provides such control of the differential dissolution rate without the need for an additive. In addition, it would be desirable to provide such polymeric materials for use as base resins of such photoresist compositions and to provide methods of forming such desirable polymers.